The present invention relates to an improvement in electrical arrangements for isolating short circuit faults and undervoltage conditions from the other part of an alarm system, such as a fire alarm system or the like.
The central controller (sometimes referred to as a control unit, also as a central panel) of a fire detection system may be required to be connected to a large number of sensors or detectors located throughout a given premises.
In general, it is know to design and manufacture fire alarm systems and devices in which, characteristically, control units or panels are provided to divide the protected premises into areas or fire zones. Each zone is monitored separately by the controller or panel for troubles and alarm conditions. This zonal arrangement allows for some precision in identifying the location of a fire alarm or a trouble condition. However, conventional systems suffer the disadvantages of the high cost of wiring involved in multiple monitoring circuits, where a more precise means for locating a fire is desired.
Because of recent demand due to stricter fire codes, as well as to rising insurance premiums and pressures from local fire departments, coupled with significant cost reductions in high technology electronic circuits, it has become more economically feasible for manufacturers of fire alarm systems and devices to develop equipment which incorporates circuitry that establishes a sophisticated communication link between the control unit or panel and the sensors or devices that are interconnected in, or connected across, appropriate transmission lines or loops.
Generally speaking, upon proper configuaration of such a sophisticated system, the control panel or unit has the ability to determine the status of any detector device as well as to pinpoint the device's exact location. These types of systems have come to be commonly referred to as "addressable fire alarm control panels". Moreover, such systems lend themselves to the elimination of complex and/or extensive wiring schemes and monitoring circiuts.
However, up until recently there has been a lack of assurance of maximum protection in the event of a hazardous condition, such as an open connection or break in the wiring; or, on the other hand, a short circuit fault in the wiring. What has been proposed heretofore, particularly in the latter case of a short circuit fault is an arrangement for detecting and isolating any such short circuit fault, but one which unfortunately could render all of the detector devices inoperative.
Reference to U.S. Pat. No. 4,528,610 makes clear the kind of circuitry that has been contemplated for isolating a section of a loop circuit from a bidirectional supply unit, in the event of a short circuit in the section; however, such an arrangement, as described, does not provide sufficient control from the control unit over the short circuit condition to insure that the possibility is eliminated of having all or substantially all of the detector devices rendered inoperative upon occurrence of the short circuit.
Accordingly, it is a primary object of present invention to provide a means which will allow a large portion of the interconnected detectors to remain operational in the event of a short circuit on a bidirectional loop ("Class A"). More specifically, it is an object of the present invention to provide selectivity in the arrangement such that only the detectors which are physically located adjacent to the shorted section of wire will be caused to switch from a low impedance path to a high impedance path.
Another object of the present invention is to provide such low impedance path during normal system operation, thus providing that the interconnected detectors' operating currents produce small line losses.
A further object of the present invention is to provide the same benefit of a low impedance path for interconnected devices that are arranged in a unidirectional configuration.
Yet another object is to provide a means to automatically disconnect detectors which are operating at line voltages that are below minimum requirements. By doing so maximum reliability is obtained.